This invention relates to electron microscopes and more particularly to scintillators for electron microscopes and a method of making scintillators. General principles of electron microscopy, and particularly scanning electron microscopy (SEM), is explained in xe2x80x9cScanning Electron Microscopyxe2x80x9d, by Postek, Howard, Johnson and McMichael, particularly pages 26, 27 and 28 (incorporated herein by reference). A beam of electrons is focussed onto a specimen producing a scattering of two types referred to as secondary and backscatterd electrons emitted outward from the specimen surface in all directions. These may be detected by a further known system as shown schematically in FIG. 1 wherein the microscope column includes a collector 1, scintillator 2, light pipe (or guide) 3 and detector base 4 together with other components forming the detector assembly. Scintillator 2 is mounted on the end of light pipe 3 covered by collector 1 supported by small screws on detector base 4 so that the scintillator is within the specimen chamber 5 when detector base 4 is mounted on a wall thereof 6 such as shown schematically in FIG. 2. This figure shows parts of the microscope that are positioned outside the specimen chamber, such as photomultiplier 7 (PMT) in case 8 mounted such as by PMT cover mount 9 on the side of detector base 4 opposite to that on which the detector assembly is mounted. A rubber light shield 10 may also be used as shown and a preamplifier case 11 is mounted at the outer end of the PMT. A collector voltage cable 12 extends through the detector base and is connected at its end 13 to collector 1. A scintillator high tension cable 14 is similarly connected at its end 15 to a ring 15 around the scintillator so that when the cables are energized the collector is positively biased to draw electrons to the scintillator. The scintillator is a thin plastic disc coated with a special phosphor and also coated with aluminum which serves as a mirror to direct the photons toward the PMT 7. The positive bias accelerates low energy secondary electrons toward the detector, but does not influence higher energy backscattered electrons. Electrons strike the scintillator and the phosphors thereon produce photons (small flashes of light), several photons being emitted, theoretically, for each incoming electron. The photons are transported through light pipe 3 from the evacuated microscope column. Light pipes are generally made of plexiglass or polished quartz for example. Photons carried by the light pipe are converted by the PMT and a photocathode (not shown) to an amplified electronic signal outside the microscope column which can then be displayed on a cathode ray tube with the brightness on the screen being proportional to the number of secondary electrons emitted from the specimen. The amplification of the signal with the PMT is far less efficient than that of the scintillator, in that noise is greatly amplified with the PMT.
Scintillators for the SEM of the type of this invention are discussed in my article xe2x80x9cScintillators For The Semxe2x80x9d, by M. E. Taylor, published in xe2x80x9cMicroscopy Todayxe2x80x9d, July, 1998 (incorporated herein by reference), which states, inter alia, that without a properly functioning scintillator images tend to be noisy, weak, or exhibit other signs of degradation. This article also states that there are three types of scintillators generally used in the SEM: organic/polymeric, phosphor powder, and crystalline (single or poly). Also, plastic scintillators are currently used less frequently mainly because they are subject to radiation damage which causes a short lifetime, although this type of scintillator has the shortest decay time (about 2.2-5 ns) and very low noise. Using a quartz substrate the scintillator material, in liquid form, is spin coated to produce a uniform thin film, which makes a more robust product and introduces a minimum of organic material to the high vacuum system. The films may be over coated with aluminum for conductivity. Various phosphor powders have been used, but the P47 line of materials appears to be most preferred. They are generally produced by settling in proprietary sollutions, with or without binders, on glass/quartz. Grain size of the phosphor, thickness of the layer, and other additives to the settlement tank can vary the results. The phosphors have a somewhat longer decay time (about 20-40 ns), but these are still within the bounds to be used at fast scan rates. They last 2-3 times longer than plastic scintillators, as long as the vacuum is clean. A contamination layer on the surface of a scintillator will reduce its efficiency.
Scintillators should be handled with the utmost care scince they are very fragile. The main problem in production is getting the material thin enough for optimum resolution. The coated surface should never be touched. They must be installed so that the active/coated side is facing toward the sample chamber and held securely in place with the scintillator retaining ring which must be in contact with the surface for optimum conductivity. Application of silver paint at this interface is no longer recommended.
Unless used in the backscatter mode, the scintillator has a 9-12 KV bias voltage applied. If arcing occurs in the area of the scintillator, damage could result. Furthermore, if scintillator material is removed in any way to produce pin holes, the underlying substrate may also charge up.
Scintillators are also discussed in my article xe2x80x9cAn Improved Light Pipe for the Scanning Electron Microscopexe2x80x9d, by M. E. Taylor, in xe2x80x9cThe Review Of Scientific Instrumentsxe2x80x9d, Vol. 43, No. 12, December 1972 (incorporated herein by reference).
U.S. Pat. No. 5,932,880 shows an SEM and a scintillator used therein in which electrodes are formed on the electron beam output plane and scintillation radiation plane and a high d.c. voltage is applied between the electrodes to control the scatter direction of an electron beam which has entered the scintillator to be in the direction of the scintillator radiation direction. In one embodiment, a transparent electrode of tin oxide, indium oxide, titanium oxide or the like is between the scintillator and a glass substrate, and the other electrode is deposited over the other side of the device and may be made of Al, Au, Ag, or Pt.
U.S. Pat. No. 5,517,033 shows an SEM using a scintillator comprised of an Al foil having a coating of scintillating material on the underside, and a mirror made of Al coated with a thin layer of Au to prevent oxidation. U.S. Pat. No. 5,536,941 shows a camera and prism assembly for electron microscopes having a scintillator of phosphor applied onto the surface of a glass prism mounted on a movable block. U.S. Pat. No. 5,491,339 shows a charged particle detection device used in addition to a scintillator for detecting a secondary electron signal from a sample using a semiconductor having on one side an oxide film covered with an Al film and a Au pad arranged around the detector in contact with the Al film, and on the inner surface of a hole therethrough an oxide film covered with a conductive film of Au, Pt, Ni, Ti or the like. All the above patents are incorporated herein by reference.
It is a principle object of the invention to provide a scintillator for an electron microscope with improved qualities over previously known scintillators by having enhanced electrical contact, a reduction in pinhole interference, and a reduction of signal to noise ratio, and by eliminating the requirement for an aluminum coating.
It is a further object of the invention to provide an improved scintillator which is easier to handle than other known scintillators and that can be recoated.
It is also an object of the invention to provide a method of making a scintillator having the above advantages over known scintillators.
The above objects are achieved by the scintillator of this invention having a generally disc shape wherein a scintillator material of phosphor, organic, or single crystal is electrically conductively connected to an optical substrate of quartz glass, polymer, or any optically clear material, or light guide of an electron microscope, by a conductive, transparent indium tin oxide coating positioned between the interfaces of the scintillator material and the substrate. A retaining ring, preferably of gold, is bonded to and positioned around the rim or outer edge of the substrate and coating and has a radially inwardly extending lip overlying the outer edge portion of the surface of the coating which is connected to the scintillator material. An electrically conductive medium of adhesive, epoxy, or solder, for example, is positioned at the interface of the overlying surface of the lip and the outer edge portion of the coating so that optimum electrical conductivity is provided between the retaining ring, the medium and the substrate.
The above objects are further achieved by the method of this invention including the steps of fabricating the substrate from glass, plastic, or quartz, for example; coating an end of the substrate with indium tin oxide (ITO) by sputtering, thermal evaporation, or electron beam evaporation, for example; fabricating the retaining ring of gold, or platinum, or brass, copper, silver, platinum or other highly conductive material coated with a non-oxidizing material such as gold or platinum to eliminate oxidation, the ring having a radially inwardly extending lip on one end; applying electrical conducting medium of adhesive, Ag epoxy or solder on the underside of the lip, or on the radially outer edge portion of the ITO coating, or both; fitting the ring onto the substrate with the lip overlying the radially outer edge portion of the ITO and bonding the ring to the substrate and ITO coating; applying scintillation material to the outer surface of the ITO coating opposite to the substrate by settlement deposition according to Stokes Law, for example, then removing excess liquid leaving deposited phosphor material on the ITO coating and lip of the ring; and removing excess phosphor from the ITO coating and the lip by wiping or scraping, for example. Optionally, the step of applying a top reflective coating on the outer surface of the scintillating material by thermal evaporation or vapor deposition, for example, could be added to reflect unwanted light.